1. Field of the Invention
The present invention relates to a communication device for receiving and transmitting OFDM signals in a wireless communication system.
2. Description of the Related Art
In wireless OFDM communication systems a communication device, as e.g. a base station, communicates with another communication device, as e.g. a mobile terminal, over a wireless communication link using OFDM signals. OFDM (orthogonal frequency division multiplex) is a multi carrier modulation method wherein information to be transmitted is mapped (e.g. by phase shift keying) onto a plurality of orthogonal subcarriers signals of different frequencies which are subsequently combined into an OFDM signal. Each subcarrier frequency defines a transmission channel in which information can be transmitted over the communication link. For more background information on OFDM it is referred, for example, to K. David, T. Benkner: “Digitale Mobilfunksysteme”, B. G. Teubner Stuttgart, 1996, S. 174-176.
The communication link causes undesired level fluctuations and distortion of the transmitted OFDM signal, e.g. due to fast fading or delay spread. Diversity methods can alleviate the adverse effects of fading. Using a plurality of antenna elements spaced apart at a certain minimum distance allows, by suitably combining reception signals received by the various antenna elements, to reliably recover the baseband information sent from another communication device even if fading occurs on one or more of the transmission paths across the communication link (receiver diversity). Transmitting one and the same transmission signal mutually delayed from several antenna elements allows to create a beam sharpened antenna pattern and to increase the received signal power at the receiver side (transmitter diversity). For more background information on receiver and transmitter diversity see, for example, EP 0 881 782 A2.
In receiving OFDM signals by array antennas or any other diversity antennas, heavy amplitude fading may occur to the entire OFDM signal at one (or more) of the antenna elements of the diversity antenna. However, it has been found that often amplitude fading does not occur to all of the subcarrier signals of an OFDM signal to the same extent. In some cases it can be observed that only particular ones of the subcarrier signals are subject to amplitude fading while other subcarrier signals and possibly even the OFDM signal as a whole do not show any severe amplitude fading.
This can be seen in FIG. 1 which schematically shows an example for the frequency selectivity of fast fading. In FIG. 1, the horizontal abscissa indicates the subcarrier number n and the vertical ordinate indicates the signal amplitude A of the aligned preamble OFDM subcarriers, wherein eight antenna elements are positioned to form a circle. Note that the dip in the center is not due to any fading but to the fact that in the Bran Hiperlan2 approach being the base for the simulation the center subcarrier is not used. By assuring that there is no DC component in the baseband signal the demodulation complexity is reduced. However, FIG. 1 clearly shows a frequency selective fading which might appear depending on the application environment.